Automatic darkening filter with offset polarizers

ABSTRACT

A protective automatic darkening filter construction  10  includes two low twist liquid crystal cells  26, 30  interspersed between a series of offset polarizers  24, 28, 32 . The resulting construction provides improved homogeneity in the dark state as viewed by the user over a large viewing angle. By reducing variations in shade, visibility through the filter may be improved.

The present invention pertains to an automatic darkening liquid crystalprotective shield or filter that can be used on a welding helmet tofilter light incident from a welder's torch.

BACKGROUND

Automatic darkening liquid crystal protective shields, also known asautomatic darkening filters, or ADFs, are often constructed from acombination of polarizing filters and layers of liquid crystals.Examples of such filters are described in U.S. Pat. Nos. 6,097,451 and5,825,441, both to Hornell and Palmer.

The ADFs change from a light (transparent) state to a darker (nearlyopaque) state in response to a control signal. For applications likewelding where protection from intense levels of incident light isdesired, the filters are normally mounted in a facemask, and the controlsignal is initiated by incident light from the welding arc. The incidentlight strikes a detector on the welding helmet which in turn sends asignal to the ADF. In this way, the filter is clear or transparent whennot subjected to the glare of the welding arc and darkens immediatelywhen exposed to such glare. This enables a welder to perform a weldingoperation and also to perform tasks outside the welding area withoutremoving the protective shield.

Conventional filters have one particular drawback, namely, the filtereffect of the liquid crystal layers is angularly dependent. In otherwords, the viewing area as seen by the user may appear to darkenunevenly. Variations in cell gap and unwanted birefringence in theadhesive layers or the polarizers may also reduce homogeneity, resultingin variations in shade over the viewing area of the welding filter.

SUMMARY OF THE INVENTION

The present invention provides a protective automatic darkening filterconstruction that addresses the problem of a non-homogeneity in theviewing area. The construction provides the user with a more even shadein the dark state over a large viewing angle. The inventive filterincludes two low twist optically rotating liquid crystal cellsinterspersed between three polarizers. The twist angle of the liquidcrystal cells is less than 90 degrees. The two outside polarizers (thefirst and third polarizers) have polarization orientations that areoffset from a normal axis to the polarization direction of the second(center) polarizer.

In one embodiment, the present invention is directed to a device thatcomprises first, second and third polarizers, and first and second lowtwist liquid crystal cells, where each liquid crystal has a twist angleof less than 90 degrees. The first low twist liquid crystal cell isdisposed between the first and second polarizers, and the second lowtwist liquid crystal cell is disposed between the second and thirdpolarizers. The polarization orientation of at least one of the first orthird polarizers is offset from a normal axis to a polarizationorientation of the second polarizer.

In another embodiment, the present invention is directed to an automaticdarkening filter, that includes a welding facemask that has a switchablefilter mounted therein. The automatic darkening filter also includes asensor that detects incident light and control circuitry that receivessignals from the sensor corresponding to the presence or absence ofincident light. The control circuitry causes a voltage to be applied tothe switchable filter in response to signal indicative of the presenceof incident light. The switchable filter includes first, second andthird polarizers, and first and second low twist liquid crystal cells,each having a twist angle of less than 90 degrees. The first low twistliquid crystal cell is disposed between the first and second polarizers.The second low twist liquid crystal cell is disposed between the secondand third polarizers. The polarization orientation of at least one ofthe first or third polarizers is offset from a normal axis to apolarization orientation of the second polarizer.

The present invention differs from known automatic darkening filters inthat at least one of the first and third polarizers are offset from anormal axis to a polarization orientation of the second polarizer. Byoffsetting the polarization orientation of at least one of the first orthird polarizers, a more homogeneous shade in the dark state may beobtained. Offsetting the polarization orientation of one or both of thefirst or third polarizers allows a small amount of light to leak throughthe filter, causing an overall softening effect that reduces variationsin shade as viewed through the filter. By reducing variations in shade,visibility through the filter may be improved. Improvements invisibility can be particularly beneficial to welders because they needto see in detail the objects on which they are working.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of an example embodiment of a protectiveswitchable filter construction 10 according to the present invention.

FIG. 2 is a diagram showing one example alignment of the polarizers andliquid crystal cells for the switchable filter 10 of FIG. 1.

FIG. 3 is a block diagram of the switchable filter 10 of FIG. 1 mountedin an automatic darkening filter 60.

FIG. 4 is a polar diagram that illustrates the filter effect atdifferent viewing angles for the switchable filter construction of FIG.1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is an exploded view of an example embodiment of a switchablefilter construction 10 that is useful for automatic darkening filterapplications. The outermost component is an interference filter 22,which filter functions in part to eliminate UV light and IR light andlimits the wavelength range. Filter 10 also includes a first polarizingfilter 24, a first optically rotating liquid crystal cell 26, a secondpolarizing filter 28, a second optically rotating liquid crystal cell30, and a third polarizing filter 32. The arrangement may optionallyalso include a so-called guest-host-cell 34.

First and second liquid crystal cells 26 and 30 are low twist liquidcrystal cells. As used in this document, the term “low twist” meanshaving a twist angle of less than 90 degrees. For example, at least oneof first and second liquid crystal cell 26 or 30 has a twist angle ofless than 90 degrees, typically zero or 1 to 89 degrees. Morespecifically, the twist angle of at least one of first and second liquidcrystal cells may be between about 20 degrees and about 85 degrees. Theliquid crystal cells 26 and 30 are provided with connectors 52 and 54,respectively, by which a control voltage may be applied.

Each of the first, second, and third polarizers 24, 28 and 32,respectively, have associated polarization orientations indicated byarrows in FIG. 1. As used in this document, the term “polarizer” means apolarizing filter that transmits light waves along one axis and absorbsthem along the other. Typically, the transmitting and absorbing axes ofpolarization are oriented at 90 degrees to each other. These axes ofpolarization, or polarization orientations, of first polarizer 24 andthird polarizer 32 may be described with respect to the orientation ofsecond polarizer 28. Second polarizer 28 has a polarization orientation47. Normal axis 56 is perpendicular to polarization orientation 47 ofpolarizer 28. In other words, the angle between normal axis 56 andpolarization orientation 47 is 90 degrees in the embodiment shown inFIG. 1. The polarization orientations of first polarizer 24 and thirdpolarizer 32 are both “offset” from normal axis 56. In other words,polarization orientation 41 of first polarizer 24 is offset from normalaxis 56 by an offset angle 40 (hereinafter first offset angle 40).Similarly, polarization orientation 53 of third polarizer 32 is offsetfrom normal axis 56 by an offset angle 46 (hereinafter second offsetangle 46). Possible offsets provided by first and second offset angles40 and 46 may be between 1 degree and 20 degrees. In one embodiment, thefirst and second offset angles 40 and 46 may be between 2 degrees and 8degrees.

FIG. 1 further shows that both first polarizer 24 and third polarizer 32are offset from normal axis 56. In some embodiments, however, only oneof first polarizer 24 or third polarizer 32 are offset from normal axis56. For example, in one embodiment, polarization orientation 41 of firstpolarizer 24 is offset from normal axis 56, while polarizationorientation 53 of third polarizer 32 is substantially parallel withnormal axis 56. In another embodiment, polarization orientation 41 offirst polarizer 24 may be substantially parallel with normal axis 56,while polarization orientation 53 of third polarizer 32 is offset fromnormal axis 56. Thus, a polarization orientation of at least one offirst polarizer 24 or third polarizer 32 is offset from normal axis 56.

First offset angle 40 may be offset from normal axis 56 in a clockwise,or positive, direction, while second offset angle 46 may be offset fromnormal axis 56 in a counterclockwise, or negative, direction. Thedirection of first and second offset angles 40 and 46 may be reversed,or, in some embodiments, they may be offset in the same direction (e.g.,either both positive or both negative). The magnitude of first andsecond offset angles 40 and 46 may be the same in some embodiments, ormay be different in other embodiments.

Liquid crystal cells 26 and 30 are “low twist” cells. That is, they havea twist angle of less than 90 degrees. A typical construction for thistype of low twist cell consists of a twisted nematic (TN) type of liquidcrystal material positioned between glass plates. The inwardly facingglass plates of the liquid crystal cells are provided with transparentelectrically conductive electrode layers (e.g., indium tin oxide layers)on which there is applied, for instance, a polyimide layer that has beentreated mechanically, such as by brushing or rubbing, in specificalignment directions. The resulting structure in the liquid crystaldefining surfaces, forces the nematic molecules to take specific angularpositions so that the molecules are twisted through their respectivetwist angle between the defining surfaces. In an electricallynon-activated state (with no voltage applied), the polarization plane isrotated as light passes through the cell and the filter becomestransparent. The orientation of the nematic liquid crystal molecules canbe controlled by applying an electric field between the definingsurfaces. Application of a voltage creates an electric field between thedefining surfaces. The nematic liquid crystal molecules align with theelectric field perpendicular to the defining surfaces, rather thanparallel to them, and the cell achieves a darkened state. Thus, when acontrol voltage is applied to the low twist cells, a filter effect isobtained. The degree of rotation of the nematic molecules may becontrolled by varying the control voltage, and thus the correspondingfilter effect may also be controlled. The result is that liquid crystalcells 26 and 30 are in a light state in the absence of an appliedvoltage is and are in a dark state in the presence of an appliedvoltage.

Guest-host-cell 34 may include a nematic liquid crystal, whose moleculesand atoms may be aligned parallel to normal axis 56 in polarizer 28. Aprepared glass surface can assist in this alignment. An inmixed pigmenthaving ordered anisotropic absorption is highly absorbent in the alignedstate. When a voltage is applied, the molecules of the nematic crystalposition themselves at right angles to the prepared surfaces, causingthe molecules of the pigment to move to positions in which the leastamount of light is absorbed. Guest-host-cell 34 therefore provides afilter effect in the absence of an applied voltage, whereas the liquidcrystal cells 26 and 30 are light transparent in the absence of anapplied voltage. In this way, filter 10 provides some minimal level ofprotection and safety to the user in the event of an unintended voltageloss.

In the embodiment shown in FIG. 1, the corresponding alignmentdirections of the liquid crystal cells 26 and 30 are arrangedsubstantially parallel to and oriented asymmetrically with respect toone another. For example, the alignment direction 45 of liquid crystalcell 26 is arranged substantially parallel to and orientedasymmetrically (i.e., in an opposite direction) with respect toalignment direction 49 of liquid crystal cell 30. Similarly, alignmentdirection 43 of liquid crystal cell 26 is arranged substantiallyparallel to and oriented asymmetrically with respect to alignmentdirection 51 of liquid crystal cell 30. This asymmetric orientation isillustrated by the opposite pointing arrows indicating the alignmentdirections 43, 45 and 49, 51 in FIG. 1.

FIG. 2 is a diagram that shows one example of how first liquid crystalcell 26, second polarizer 28, and second liquid crystal cell 30 may bearranged with respect to one another. FIG. 2 shows how the normal axis56 and polarization orientation 47 of second polarizer 28 areperpendicular to each other as described above. Bisector 58 bisects theangle between polarization orientation 47 and normal axis 56. Thus,bisector 58 forms a 45 degree angle with polarization orientation 47 andnormal axis 56 in this example. To optimize performance in the lightstate, alignment directions 45, 43 and 49, 51 of liquid crystal cells 26and 30, respectively, may be arranged symmetrically with respect tobisector 58. For example, if the twist angle of liquid crystal cells 26and 30 is equal to 70 degrees, then alignment directions 45 and 43 wouldbe located symmetrically 35 degrees about bisector 58. Similarly,alignment directions 49 and 51 would also be located symmetrically 35degrees about bisector 58. Thus, in this example, the angle 57 betweenalignment directions 43, 51 and bisector 58 is 35 degrees, as is theangle 59 between alignment directions 45, 49 and bisector 58. Statedanother way, the angle between normal axis 56 and alignment directions43, 51 would be 10 degrees, and the angle between normal axis 56 andalignment directions 45, 49 would be 80 degrees in this example.

Although a symmetric orientation may optimize the light state, otherorientations may also be used, and the present invention is not limitedto the arrangements shown and described herein with respect to FIGS. 1and 2. Many other configurations are also possible without departingfrom the scope of the present invention.

FIG. 3 is a block diagram of an automatic darkening filter (ADF) 60.Automatic darkening filter 60 includes a switchable filter 10 that hasoffset polarizers of the type described above with respect to FIGS. 1and 2. Switchable filter 10 is mounted in a welding facemask 66 thatwould be worn by the user during a welding procedure or other situationwhere protection of the type provided by switchable filter 10 isdesired. ADF 60 also includes a sensor 64 for detecting light incidentupon the front surface of filter 10, such as a welding arc. Controlcircuitry 62 receives signals from sensor 64 corresponding to thepresence or absence of incident light and causes corresponding controlvoltages to be applied to filter 10, thus controlling the degree ofshade provided by filter 10. When the presence of a welding arc or othersource of incident light is detected by sensor 64, for example, controlcircuitry 62 may cause a control voltage to be applied to liquid crystalcells 26 and 30 while eliminating the voltage to guest-host cell 34.This causes the filter to darken and protect the user from the glare ofthe welding arc. In the absence of a welding arc or other source ofincident light, control circuitry 62 may reduce or eliminate the appliedvoltage to liquid crystal cells 26 and 30, thus causing the filter tobecome more open to light. This enables a welder to perform a weldingoperation and also to perform tasks outside the welding area withoutremoving the protective facemask. In addition, the filter constructiondescribed herein results in increased homogeneity in the dark state asseen by the user over a large angular range.

The switchable filter 10, sensor 64, and control circuitry 62 aretypically supported on a helmet shell of a welding facemask 66 as aunit, typically a replaceable unit that is mounted in the shell directlyin front of the wearer's eyes when the helmet is worn by the user. Theunit may take the form of a rectangular (or other shaped) frame orhousing that supports the filter, sensor, and circuitry. Examples ofhelmet shells may be seen, for example, in U.S. Pat. Nos. 6,185,739,5,533,206, 5,191,468, 5,140,707, 4,875,235, and 4,853,973. The weldinghelmets also can have clean air supplied to their interior and thus mayinclude a face seal to separate a breathing zone from the ambient air.An example of such a face seal is shown in U.S. patent application Ser.Nos. 10/987,512, 10/987,641, 10/988,789, 29/217,155, 29/217,153,29/217,154, 29/217,107, 29/217,156.

One measure of the filter effect is the so-called Shade Number. ShadeNumber, S is related to luminous transmittance, TL (expressed as afraction) by the following equation:S=1+7/3×10 log (1/TL).

A filter arrangement of the type described herein may change from afilter effect of about Shade 3.3 in the light state to values rangingfrom about Shade 9 to about Shade 13 in the dark state. The filtereffect may change by varying the applied voltage from about 2.0 V toabout 4.5 V.

FIG. 4 is a polar diagram 80 showing the Shade value at various viewingangles. FIG. 4 was measured using a switchable filter construction likethat shown in FIG. 1, with the first polarizer 24 having an offset angleof 6 degrees measured in a clockwise direction, third polarizer 32having an offset angle of 6 degrees measured in the counterclockwisedirection, and liquid crystal cells 26 and 30 having a twist angle of 80degrees. The outermost circle on polar diagram 80 represents a deviationof 30 degrees from a perpendicular angle of incidence (indicated at thecenter of the diagram). The diagram was produced in a stepping machinewith the inclination taken in steps of 2.5 degrees and with an azimuthstep of 10 degrees. Domains where every gray level represents an equalfilter effect have been produced in Shade steps of 0.5. The DIN shadescale is indicated by reference numeral 82. Because the twist angle ofthe liquid crystal cells 26 and 30 differs from 90 degrees, and becausethe orientations of the first and third polarizers 24 and 32 are offset,the filter effect in the dark state is more uniform over varying anglesof view than conventional filter constructions.

The measurements involved the use of two identical liquid crystal cells.In that type of embodiment, both liquid crystal cells 26 and 30 can bedriven with the same control voltage, and this control voltage can bevaried to produce different densities and thus different degrees ofshade in the dark state. This provision may simplify the electronicsthat are required.

The liquid crystal cells 26 and 30, however, need not be identical. Inone embodiment, for example, a 90 degree twist liquid crystal cell andlow twist cell having a twist of between 20 degrees to 85 degrees may bearranged between offset polarizers. In another embodiment, two low twistcells each having different degrees of twist may be arranged betweenoffset polarizers. Different cells may be combined so as to achieve anoptimized total solution, depending upon the desired end result. Forinstance, it is possible to combine symmetrically and asymmetricallymounted liquid crystal cells, cells of different twist angles andthicknesses, etc.

All of the patents and patent applications cited above, including thosecited in the Background Section, are incorporated by reference into thisdocument in total.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A device, comprising: a first, second and third polarizers; and firstand second low twist liquid crystal cells, each having a twist angle ofless than 90 degrees, wherein the first low twist liquid crystal isdisposed between the first and second polarizers, and wherein the secondlow twist liquid crystal is disposed between the second and thirdpolarizers; and further wherein a polarization orientation of at leastone of the first or third polarizers is offset from a normal axis to apolarization orientation of the second polarizer.
 2. The device of claim1, wherein the twist angles of the first and second liquid crystal cellsare between 20 degrees and 85 degrees.
 3. The device of claim 1, whereinthe first and second liquid crystal cells are substantially identical,and wherein the control voltage applied to each of the first and secondliquid crystals is substantially identical.
 4. The device of claim 1,wherein a polarization orientation of the first polarizer is offset fromthe normal axis by a first offset angle.
 5. The device of claim 4,wherein the first offset angle is between 1 degree and 20 degrees. 6.The device of claim 1, wherein a polarization orientation of the thirdpolarizer is offset from the normal axis by a second offset angle. 7.The device of claim 6, wherein the second offset angle is between 1degree and 20 degrees.
 8. The device of claim 1, wherein a polarizationorientation of the first polarizer is offset from the normal axis, andwherein a polarization orientation of the third polarizer is offset fromthe normal axis.
 9. The device of claim 1, wherein a polarizationorientation of one of the first and third polarizers is offset from anormal axis to a polarization orientation of the second polarizer by afirst offset angle measured in a clockwise direction, and wherein apolarization orientation of the other one of the first and thirdpolarizers is offset from the normal axis to the polarizationorientation of the other of the second polarizer by a second offsetangle measured in a counterclockwise direction.
 10. The device of claim9, wherein the first and second offset angles have magnitudes that aresubstantially equivalent.
 11. The device of claim 9, wherein themagnitudes of the first and second offset angles are between 1 degreeand 20 degrees.
 12. The device of claim 1, wherein the first low twistliquid crystal cell includes corresponding first and second alignmentdirections, and wherein the first and second alignment directions arearranged substantially symmetrically with respect to a bisector of thepolarization orientation of the second polarizer and a normal axis tothe polarization orientation of the second polarizer.
 13. The device ofclaim 1, wherein the second low twist liquid crystal cell includescorresponding first and second alignment directions, and wherein thefirst and second alignment directions are arranged substantiallysymmetrically with respect to a bisector of the polarization orientationof the second polarizer and a normal axis to the polarizationorientation of the second polarizer.
 14. An automatic darkening filterunit that comprises the device of claim 1, and further includes a sensorfor sensing incident light from a welder's torch and a control circuitrythat generates and delivers a signal to the device to cause the deviceto darken in response to the incident light.
 15. A welder's helmet thatcomprises a shell and the automatic darkening filter unit of claim 14,the automatic darkening filter unit being supported by the shell suchthat the unit is disposed in front of a wearer's eyes when the weldinghelmet is being worn.
 16. The welder's helmet of claim 15, wherein theautomatic darkening filter unit is replaceable.
 17. A device,comprising: a welding facemask; a switchable filter mounted in thewelding facemask; a sensor to detect incident light; and controlcircuitry that receives signals from the sensor corresponding topresence or absence of incident light and that causes a voltage to beapplied to the switchable filter in response to signal indicative of thepresence of incident light; wherein the switchable filter includes:first, second and third polarizers; and first and second low twistliquid crystal cells, each having a twist angle of less than 90 degrees,wherein the first low twist liquid crystal cell is disposed between thefirst and second polarizers, and wherein the second low twist liquidcrystal cell is disposed between the second and third polarizers; andfurther wherein a polarization orientation of at least one of the firstor third polarizers is offset from a normal axis to a polarizationorientation of the second polarizer.